The Free Prebyterian online copy of the Westminster Confession quotes three verses in support of the statement :
neither is God the author of sin,b
Free Presbyterian Church Documents
The first two are from James :
Let no man say when he is tempted, I am tempted of God: for God cannot be tempted with evil, neither tempteth he any man:
James 1:13
Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning.
James 1:17
The third is from John :
This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all.
I John 1:5
I was not certain but @curiousdanni notes below that these texts are original to the Westminster Confession and are not added by the Free Presbyterian website.